In info.magnolia.module.exchangesimple.BaseSyndicatorImpl.transportActivatedData(ActivationContent, URLConnection, String), a timestamp of current author time is gathered, and later on sent to public instances together with the activation request. The activation request is then refused by the public instance if the public's current time diverges from this author timestamp by more than 30s, presumably in order to verify author and public clocks are in sync.
Unfortunately, the timestamp on author is created before the temporary file with activation data is created in Transporter.transport(). Given a recursive activation with huge amounts of binary data (multiple GB) plus some load on author and public, these 30s can too easily be exceeded.
The timestamp on author should be created as shortly as possible before the actual sending of the activation HTTP request. Otherwise, the clock sync check will fail when creating the temp file takes longer than 30s, which probably isn't desired.